Various techniques have been developed for producing polymers consisting predominately of ethylene. The high pressure free radical process produces the polymer that has come to be known as low density polyethylene (LDPE) results in polymer chains having multiple branches many of which are indistinguishable from the main polymer chain. Techniques for low pressure polymerization have been developed using modifications of transition metal coordination catalyst systems such as those originally discovered by Ziegler and Natta. Such catalyst are capable of producing high density polyethylene (HDPE), i.e polyethylene having a density of greater than about 0.94 g/cc, medium density polyethylene (MDPE), and linear low density polyethylene (LLDPE), i.e. polyethylenes having densities generally no greater than about 0.925 g/cc in solution, gas phase, and slurry phase polymerizations. The phrase "transition metal coordination catalyst systems" is used to include catalyst system involving the combination of a transition metal compound of a metal such as Ti, V, and Zr used in combination with an organoaluminum cocatalyst, generally an organoaluminum alkyl.
Another technique for producing polyethylene involves the use of the Phillips chromium catalyst system.
The various types of polyethylenes find different applications as a result of differences in the structure of the polymer chains. For example it is generally recognized that LLDPE which have a density of no more than 0.925 g/cc are superior to HDPE in transparency, impact resistance, and environmental stress crack resistance (ESCR) and are superior to LDPE in impact resistance and creep resistance. Such LLDPE is however generally inferior to LDPE in transparency, melt processing, and melt elasticity. But LDPE on the other hand has lower ESCR and lower mechanical strength than LLDPE of similar density.
The various polymerization techniques for producing HDPE and LLDPE likewise result in polyethylene polymers having various types of polymer microstructure. The transition metal coordination catalyst systems generally produce polymer chains having varying lengths so that the polydispersity index, i.e. heterogenity index or molecular weight distribution, is generally greater than 4. The Phillips chromium catalyst systems generally produces polyethylenes having an even broader molecular weight distribution. When the chromium and transition metal coordination catalyst systems are employed for copolymerizing ethylene with alpha olefms containing 3 to 10 carbon atoms the distribution of the comonomer among the various polymer chains has been noted to also vary, i.e. the polymer chains can contain different levels of comonomer insertion and the distribution of the comonomer insertion within a given chain can also vary in chains of differing lengths. Particularly, it has been noted in the polymers produced with the chromium and transition metal coordination catalyst systems the comonomer insertion is much less in the longer polymer molecules. The phenomenon regarding the greater incorporation of comonomer in the lower molecular weight chains has also been documented in Exxon's U.S. Pat. No. 5,382,630, note especially column 1, lines 46-52; Hosoda, S., Polym. J., 20, 389 (1988): and Randal and Hsieh, NMR and Macromolecules, ACS Symposium Series 247, Chapter 9 (1984).
Also the copolymers produced using the Phillips Cr catalyst systems generally have significant amounts of terminal vinyl groups, i.e. about 1 vinyl end group for each saturated end group, whereas the polymers produced with the transition metal coordination catalyst systems of comparable MI generally have a ratio of terminal vinyl groups to saturated end groups of less than 1/4, if they contain any terminal vinyl groups at all. These variances in the structure of the polymer chains can have significant effects on the physical properties of the polymer and thus can be of great importance to determining the applications for a particular polymer.
At a given M.sub.w and M.sub.w /M.sub.n the density of LLDPE and HDPE polymers is primarily controlled by the introduction of short chain branches into the polymer. Some catalyst systems have been noted to result in the formation of branches without the employment of comonomer. More typically the short chain branches are introduced by the employment of a minor amount of an alpha olefin comonomer having 3 to 8 carbon atoms. The amount of comonomer incorporated is generally less than 20 mole percent, more typically less than 10 mole percent for the typical LLDPE. Obtaining lower densities involves introducing more short chain branches by using more comonomer. In order to obtain ethylene polymers having a density of about 0.915, it has been noted that for chromium catalysts and transition metal coordination catalyst systems typically the ratio between the mole percent of comonomer, as reflected by short chain branching in the polymer, to the density is at least about 5.
One object of the present invention is to provide polyethylenes that are particularly useful for making films of unusual clarity at a given density, said polyethylenes having a very narrow molecular weight distribution and no detectable amounts of long chain branching.
Another object of the present invention is to provide polyethylene having ethyl branches wherein the level of ethyl branches is substantially the same regardless of the molecular weight of the polymer molecules.
Another object of the present invention is to provide certain types of polyethylene having ethyl branches wherein the molar percent of ethyl branches is substantially the same for molecular fractions across the molecular weight distribution of the polymer. One type of such polyethylene is substantially free of any other type branches. Another type of such polyethylene has branches having 3 or more carbons while at the same time having ethyl branches wherein the molar percent of the ethyl branches is substantially the same for molecular fractions across the molecular weight distribution of the polymer. In a preferred embodiment the polyethylene is substantially free of branches having 6 or more carbon atoms.
Another object of the present invention is to provide certain polyethylene having significant amounts of terminal vinyl groups, a very narrow molecular weight distribution, and a lower density than one would normally expect for the amount of branching due to comonomer insertions in the polymer chains.
Another object of this invention is to provide certain narrow molecular weight polyethylenes having an unusually low its density for the amount of short chain branching at a given molecular weight.
Another object is to provide certain copolymers of ethylene wherein the copolymer is produced from olefins limited to ethylene and alpha olefin comonomers having either 3 or 5 or more carbon atoms per molecule.
Other aspects, objects, and advantages of the present invention will be apparent from the discussion which follows.